It is known that the presence of humic substances and nitrates contained in particular in natural, particularly stagnant, water prohibits direct consumption thereof. In fact, the humic substances develop unpleasant odours and are potentially carcinogenic. These humic substances are products of decomposition of plant or animal waste; they are in the form of polymers with polyaromatic structural units, of high molar mass (higher than 200,000 for the humic matter, between 2,000 and 150,000 for the humic acids, less than 2,000 for the fulvic acids). They are generally associated with nitrates of which the toxicological risks are well known.
Different ways have been sought to eliminate the humic substances and nitrates from natural water, particularly to render it drinkable.
Among possible ways, the technique employing ion exchange was not successful, despite the presence of carboxylic functions in the structure of the humic substances. In fact, in the field of ion exchange, it is well known that these substances are poisons for the resins, i.e. they are fixed very rapidly on the surface of the resin and, due to their high molar mass, the superficially fixed molecules prevent the diffusion of other molecules in the internal part of the resin.
Tests run by Applicants have shown that, with respect to the total exchange capacity of the resins, the effective exchange efficiency vis-a-vis the humic substances is only of the order of three per cent. These tests were carried out with five different types of resins, which gave very similar results, namely from 9 to 11 mg of humic substances retained per gram of resin for 300 mg potentially fixable taking into account the capacity of the resins.
Another drawback has been observed. The humic substances fixed on the surface of the resins are not extractable and the resins are therefore not regeneratable. This may be due to irreversible phenomena of adsorption of the humic substances by the resins produced in addition from the ion exchange proper.